1. Field of the Invention
The present invention relates to a process for preparing acetic acid by gas-phase oxidation of saturated C.sub.4 -hydrocarbons and their mixtures with unsaturated C.sub.4 -hydrocarbons using a coated catalyst, and also to a coated catalyst for preparing acetic acid by gas-phase oxidation of saturated C.sub.4 -hydrocarbons and their mixtures with unsaturated C.sub.4 - hydrocarbons.
2. The Prior Art
It is known that acetic acid can be prepared by gas-phase oxidation of C.sub.2 -, C.sub.3 - and C.sub.4 -molecules with the aid of a catalyst. However, no process which is fully satisfactory from economic and process engineering points of view has yet been found.
DE-B 1,279,011 describes a process for preparing acetic acid by catalytical gas-phase oxidation of butene by oxygen using catalysts comprising aluminum vanadate and titanium vanadate. These catalysts are prepared by precipitation of the mixed oxides from the corresponding solutions and the mixed oxides can, if desired, be mixed with inert materials such as silica. The catalyst is used in fluidized-bed reactors as a finely divided powder. A disadvantage of such catalysts is the high degree of total oxidation.
To improve the yield obtained by means of such catalysts, DE-A 2,016,681 proposes that the catalysts be pretreated with an oxidizing agent before calcination.
DE-A 2,354,425 (U.S. Pat. No. 3,954,857) proposes treating the calcined titanium-vanadium mixed catalyst with hydrochloric acid to improve the selectivity. The catalysts are used as fully active catalysts, if desired in admixture with inert support materials such as silica.
A further way known from the prior art of improving the activity of titanium-vanadium mixed catalysts in the gas-phase oxidation of butenes to acetic acid is the use of TiO.sub.2 in a defined crystal form or with a defined surface area.
DE-A 2,026,744 (U.S. Pat. No. 3,917,682) describes Ti-V mixed catalysts whose TiO.sub.2 component is predominantly in the form of rutile. The catalysts can be used in powder form or after being pressed to form shaped bodies.
U.S. Pat. No. 4,448,897 discloses Ti-V catalysts for butene oxidation which comprise TiO.sub.2 having a BET surface area of more than 40 m.sup.2 /g. The catalysts are likewise used in powder form or as compacts.
It is also known from the prior art that the selectivity of Ti-V catalysts in the oxidation of butene can be improved by completely or partly replacing the titanium dioxide by other metal oxides.
For example, DE-A 2,110,876 (GB-A 1,333,306) describes catalysts comprising oxides of molybdenum, tin and vanadium as active components. The catalysts are used in powder form and the mixed oxide catalyst can also, if desired, be applied to finely divided support materials such as silicon dioxide.
U.S. Pat. No. 4,146,734 discloses the use of vanadium mixed oxides which are doped with cerium and further transition metal oxides. The catalyst is used as finely divided powder, but can also be applied as precipitate to finely divided, inert supports.
DE-A 2,235,103 discloses Ti-V mixed oxide catalysts for the gas-phase oxidation of butenes in the form of supported catalysts in which a preformed porous support is impregnated with the mixed solution of the catalyst components.
In all these processes, use is made of catalysts in which the active components are employed as such as powder or compacts. Also the active components can be employed in the form of powder or compacts in which they are diluted with finely divided support materials. For the purposes of the present invention, bulk catalysts also include porous supports which have been impregnated right through with active component as described in DE-A 2,235,103. Also, in this case too, the entire catalyst volume is catalytically active. Disadvantages of fully active catalysts are the high degree of total oxidation and the difficulty of controlling the oxidation reaction at high conversions.